upfloor

plural upfloors

1. architectureAn upper storey, especially a gallery or arcade above the arches of the nave, choir, and transepts of a church.

   […] and, as the upfloor was used for the transaction of business, attended by many witnesses, we may suppose that it was a large, wide, lofty upper story, such as is found in many early Norman minsters.

   1878, Edward Hungerford Goddard, The Wiltshire Archaeological and Natural History Magazine

   1 more example

   At Westminster, a part of the shafts set about the columns support the arches and groinings of the ailes, while others in the cluster ascend the upfloors and sustain the munions of the vaulting of the nave.

   1819, Charles Clarke, Architectura Ecclesiastica, page 8